This invention relates to a method and apparatus for freezing products and, more particularly but not exclusively, is concerned with a method and apparatus for freezing foodstuffs.
The use of liquid nitrogen to freeze foodstuffs has increased dramatically over the past 30 years. The improvement in the quality of the frozen food is well known. However, whilst liquid nitrogen is now used for freezing premium food products its cost prevents it being used for freezing those foodstuffs which do not command a premium price. These foodstuffs are typically frozen using mechanical refrigeration.
Over the years many attempts have been made to reduce the quantity of liquid nitrogen required to freeze a given foodstuff and gradually it has become economically viable to use liquid nitrogen to freeze an increasing range of foodstuffs.
The present invention aims to continue this trend. In particular, in existing cryogenic tunnel freezers the food is carried through the freezer on belts which are normally made of stainless steel. These belts typically weigh from 8 kg per square meter of surface area to 19 kg per square meter of surface area according to the products being frozen. Typically, the smaller the product the higher the weight per square meter of surface area because of the need to provide a small mesh to inhibit the product falling through the belt.
The belts are normally made of metal since metal detectors are normally positioned downstream of tunnel freezers and it is very easy to detect whether any metal from the belt, or elsewhere in the tunnel freezer accidentally enters a product. Belts of plastics material have been used but their use has generally been discontinued because plastics materials tend to become embrittled at low temperatures and there is no practical method of detecting small fragments of plastics material in the frozen product.
The present invention is based on the observation that the weight per square meter of surface area of a metal belt can have a profound effect on the consumption of liquid nitrogen, particularly as the belt speed increases.
According to one aspect of the present invention there is provided a tunnel freezer having a metal belt for carrying product therethrough, characterised in that the weight of said belt is from 1 kg per square meter of surface area to 6 kg per square meter of surface area.
Preferably, said belt has a weight of from 2 kg per square meter of surface area to 4 kg per square meter of surface area.
More, preferably, said belt has a weight of from 2.2 kg per square meter of surface area to 3.1 kg per square meter of surface area.
Advantageously, said belt has a weight of from 2.6 kg per square meter of surface area to 2.9 kg per square meter of surface area.
The present invention is particularly useful in combination with those tunnel freezers which are provided with a spray bar and a tray which, in use, contains liquid cryogen and into which the laden belt sags.
The present invention also provides a method of operating a tunnel freezer having an inlet, an outlet and a belt to convey products to be frozen between said inlet and said outlet, characterized by adjusting the speed of the belt to maintain the temperature of said belt adjacent said inlet substantially constant.
There is also provided a tunnel freezer for carrying out this method, which is characterised by means to vary the speed of the belt to maintain the temperature of said belt adjacent said inlet substantially constant.
In one embodiment said means comprises a temperature sensor located at or adjacent the inlet and arranged to generate a signal indicative of the temperature of said belt adjacent the inlet of said freezing tunnel and a controller arranged to, in use, vary the speed of said belt in response to the signal received from said temperature sensor.
For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings, in which: